Self-diagnosing FMCW radar level gauge

ABSTRACT

A self-diagnosing FMCW radar level gauge and a method for providing self-diagnosing with a radar level gauge is provided in a radar level gauge comprising a transceiver, a mixer, a signal propagating device and a signal propagation path connecting the transceiver and the signal propagating device, a filter arrangement and processing circuitry. The filter arrangement provides a filtered intermediate frequency signal. The transceiver outputs either a diagnostic sweep configured such that a reference echo from the signal propagation path is detectable in said filtered intermediate frequency signal, or a measurement sweep configured such that the reference echo is suppressed in the filtered intermediate frequency signal and that a surface echo is detectable. The processing circuitry is configured to self-diagnose the radar level gauge based on the reference echo, and to determine the distance to the surface based on the surface echo.

FIELD OF THE INVENTION

The present invention relates to a self-diagnostic FMCW radar levelgauge and to methods for using diagnostic sweeps to provideself-diagnosis of a FMCW radar level gauge.

BACKGROUND OF THE INVENTION

Radar level gauges are in wide use for measuring the filling level of aproduct contained in a tank. Radar level gauging is generally performedeither by means of non-contact measurement, whereby transmittedelectromagnetic signals are radiated towards the product contained in atank, or by means of contact measurement, often referred to as guidedwave radar (GWR), whereby transmitted electromagnetic signals are guidedtowards and into the product by a probe acting as a waveguide.

Radar level gauges are often classified as either pulsed system orFMCW-systems. In FMCW-systems, a signal with varying frequency istransmitted towards the surface and the distance is determined based onthe frequency (and/or phase) difference between a transmitted signal anda simultaneously received signal. The transmitted signal is reflected bythe surface of the contents in the tank (or by any other impedancetransition) and an echo signal, which has been delayed a certain time,is returned to the gauge. The echo signal is mixed with the transmittedsignal to generate a mixer signal, having a frequency equal to thefrequency change of the transmitted signal that has taken place duringthe time delay. For a linear sweep, this difference frequency, alsoreferred to as an intermediate frequency (IF), is proportional to thedistance to the reflecting surface. The mixer signal is often referredto as an IF signal. The IF-signal will also comprise frequencycomponents from reflections in the antenna and similar near-zone echoes.These near-zone echoes are very strong due to them occurring close tothe transceiver, and thus making it hard to detect the surface echo.Hence, radar level gauges have filters which filter the IF-signal priorto any sampling, thereby suppressing frequency components which are notrelated to the surface in order to get good measurement data.

Radar level gauges are in many cases used for applications wheremalfunction of the radar level gauge could result in dangeroussituations, and therefore radar level gauges must be extremely reliable.Various measures are taken to ensure the reliability of radar levelgauges, and to thereby reduce the risk of dangerous situations. Forinstance, by performing a self-diagnostic function the radar level gaugemay be proof tested to ensure that it is working properly. One way toperform a self-diagnostic function is to measure a reference echo, thiswill proof test the components of a radar level gauge which are relatedto generating, guiding, filtering, amplifying, transmitting and/orreceiving electromagnetic signals, also known as the microwave chain.

U.S. Pat. No. 5,614,911 describes a FMCW radar level gauge whereproblems such as the formation of a deposit on the antenna, or othertrouble such as damage or loss of the antenna may be detected. Theproblems are detected by first storing an undisturbed referencefunction, i.e. a tank spectrum, before the radar level gauge is put intoactual operation. During actual operation, the measured echo function iscompared to the stored undisturbed echo function and any differences areanalyzed and evaluated to recognize the formation of deposits or othertrouble. Similarly, US patent application 2006/0015292 is atime-domain-reflectometry (TDR) radar level gauge which may detectaccretion of material on the antenna or malfunction of the electronicsof the TDR radar level gauge. A constant measurement of the distance toa surface over a predetermined time interval is used to recognizeaccretion of material on the antenna or malfunction of the electronicsof the TDR radar level gauge such as a short-circuit in the couplingbetween the antenna and the electronics. However, as antenna echoes aredifferent for each antenna and as they degrade and change over time theyare not suitable to use for a diagnostic functionality.

GENERAL DISCLOSURE OF THE INVENTION

With regards to the above-mentioned desired properties of a radar levelgauge, it is a general object of the present invention to enable prooftesting of an FMCW radar level gauge by providing a reference echowithout adding components or costs to the radar level gauge.

The present invention is based upon the realization that by configuringthe relation between distance and frequency in a frequency sweep for anFMCW radar level gauge, an adapted frequency sweep is provided wherebyechoes in the signal propagation path from the transceiver to a signalpropagation device are not discriminated. Thereby, the echoes in thepropagation path will provide a detectable reference echo in order toenable the radar level gauge to self-diagnose the microwave chain.

According to a first aspect of the present invention, these and otherobjects are achieved through a self-diagnosing FMCW radar level gaugefor measuring a distance to a surface of a product contained in a tank.The radar level gauge comprises a transceiver arranged to generate andtransmit an electromagnetic transmit signal in the form of a frequencysweep, and a signal propagating device and a signal propagation pathwhich connects the signal propagating device to the transceiver. Thesignal propagation path and the signal propagating device are configuredto guide the electromagnetic transmit signal towards the surface, andreturn an echo signal including reflections from the surface and animpedance transition in the signal propagation path. The radar levelgauge further comprises a mixer connected to the transceiver, andconfigured to mix the echo signal with a portion of the electromagnetictransmit signal to provide an intermediate frequency signal. A filterarrangement is connected to the mixer and configured to filter theintermediate frequency signal in order to provide a filteredintermediate frequency signal, and processing circuitry is connected tothe filter arrangement and configured to process the filteredintermediate frequency signal. The frequency sweep is one of adiagnostic sweep and a measurement sweep, where the diagnostic sweep isconfigured such that a reference echo is detectable in the filteredintermediate frequency signal, the reference echo being indicative of adistance to the impedance transition. The measurement sweep isconfigured such that the reference echo is suppressed in the filteredintermediate frequency signal, and that a surface echo is detectable inthe filtered intermediate frequency signal, the surface echo beingindicative of a distance to the surface. The processing circuitry isconfigured to self-diagnose the radar level gauge based on the referenceecho, and to determine the distance to the surface based on the surfaceecho.

According to a second aspect of the present invention, the objects arealso achieved by a method for providing self-diagnosis of a FMCW radarlevel gauge for measuring a distance to a surface of a product containedin a tank. The method comprises generating an electromagnetic transmitsignal in the form of a frequency sweep, and guiding the electromagnetictransmit signal via a signal propagation path and a signal propagatingdevice towards the surface. The method further comprises returning anecho signal including reflections from the surface and an impedancetransition in the signal propagation path, and mixing the echo signaland the transmit signal to provide an intermediate frequency signal. Theintermediate frequency signal is filtered to provide a filteredintermediate frequency signal. The frequency sweep is one of adiagnostic sweep and a measurement sweep, where the diagnostic sweep isconfigured such that a reference echo is detectable in the filteredintermediate frequency signal, the reference echo being indicative of adistance to the impedance transition. The measurement sweep isconfigured such that the reference echo is suppressed in the filteredintermediate frequency signal, and that a surface echo is detectable inthe filtered intermediate frequency signal, the surface echo beingindicative of a distance to the surface. The method further comprisesprocessing the filtered intermediate frequency signal to self-diagnosethe radar level gauge based on the reference echo, and to determine thedistance to the surface based on the surface echo.

The diagnostic sweep is such that the filtered intermediate frequencycomprises echoes from impedance transitions in the propagation path fromthe transceiver to the signal propagation device.

The signal propagation path should be construed as the connectionbetween a microwave source comprised in the transceiver and the signalpropagating device. Thus, impedance transitions along this path includea transition created by the connection to the signal propagating devicee.g. an antenna.

The echoes from the propagation path are much closer than the echoesbelow the signal propagation device, and will be much stronger than theechoes from e.g. the surface of the product. Therefore in a normal FMCWradar level gauge those strong echoes, which comprise low frequencies,will be filtered out or suppressed by the filter arrangement. Otherwisethe radar level gauge would not be able to detect the echo received fromthe surface and hence not be able to determine the distance to thesurface. At least one additional advantage with the present invention isthat the reference echo is provided from the propagation path and willbe the same regardless of the signal propagation device used or otherconditions. Yet another additional advantage is that radiated power ineach sweep is efficiently used for the purpose at hand, a measurementsweep will measure the distance to the surface, whereas the diagnosticsweep will enable the diagnostic function. Therefore the presentinvention provides a reliable and simple way to proof test a FMCW radarlevel gauge without adding components or large costs.

The measurement sweep should be understood as an ‘ordinary’ FMCW sweepin which the echoes from the propagation path are suppressed by thefilter arrangement.

To self-diagnose the radar level gauge, the processing circuitry mayfurther comprise a self-diagnosis block configured to compare thereference echo against a stored reference echo profile. It should beunderstood that by configuring the diagnostic sweep the reference echowill be detectable at an expected position, i.e. distance, and/orexpected amplitude. Therefore the stored reference echo profile mayadvantageously comprise information regarding an expected distanceand/or an expected amplitude of the reference echo for a certainconfiguration of the diagnostic sweep. Hence, if the reference echo doesnot appear at an expected distance and/or with an expected amplitude,the self-diagnosis block of the processing circuitry can self-diagnosethe radar level gauge and determine whether the microwave chain isfunctioning in an expected manner.

In order to enable the filtered intermediate frequency signal tocomprise echoes from the propagation path, the diagnostic sweep may beconfigured with a shorter sweep time than the measurement sweep. Invarious embodiments the same effect may be enabled by configuring thediagnostic sweep with a larger sweep bandwidth than the measurementsweep. A combination of the two configurations is also possible. Hence,it should be understood that increasing the frequency per time unit inthe frequency sweep allows the echoes in the propagation path to bedetected. For some embodiments the sweep time of the diagnostic sweepmay be two times shorter than the sweep time of the measurement sweep.For other embodiments the sweep time of the diagnostic sweep may be fourtimes shorter than the sweep time of the measurement sweep. Accordingly,in other exemplary embodiments of the present invention the diagnosticsweep may be arranged to have a diagnostic sweep bandwidth which is twoor four times greater than the measurement sweep bandwidth.

The filter arrangement may advantageously be configured to suppress theintermediate frequency signals which correspond to the reference echoesin a measurement sweep, or amplify the echo corresponding to thedistance (and thus reference echo) in a diagnostic sweep. Thus, invarious embodiments the filter arrangement may comprise high passfilters and low pass filters. The filtered intermediate frequency signalmay therefore be seen as compensated in at least amplitude regarding thedistance from the transceiver. The power radiated from anelectromagnetic source decays according to an inverse square law,without the compensation for the distance from the filter arrangementthe signal from the propagation path and the antenna connection would beorders of magnitude larger than the echo from the surface. Furthermore,a low pass filter may enable the filter arrangement to anti-alias i.e.filter out high frequencies, such as frequencies higher than half thesampling frequency of the processing circuitry. In embodiments where alow pass filter is present the reflection from the surface may befiltered out and thus not measurable during the diagnostic sweep.

Further features of, and advantages with, the present invention willbecome apparent when studying the appended claims and the followingdescription. The skilled person realize that different features of thepresent invention may be combined to create embodiments other than thosedescribed in the following, without departing from the scope of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing anembodiment of the invention.

FIG. 1 is a schematic representation of a self-diagnosing FMCW radarlevel gauge according to a first embodiment of the present invention.

FIG. 2 is a schematic representation of the measurement electronics ofFIG. 1.

FIG. 3 is a schematic representation of the filter arrangementcharacteristics according to one embodiment of the invention.

FIGS. 4A-C are schematic representations of a tank spectrum i.e. aFast-Fourier transformed filtered intermediate frequency signalaccording to one embodiment of the present invention.

FIGS. 5A-C are schematic views of measurement sweeps and diagnosticsweeps according to embodiments of the invention.

FIG. 6 is a flow chart of an embodiment according to another embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present description, embodiments of the present invention aremainly described with reference to a radar level gauge having a freepropagating antenna for radiating and capturing an electromagneticsignal. It should be noted that this by no means limits the scope of theinvention, which is equally applicable to other signal propagatingdevices, including other free propagating antennas such as a rodantenna, a patch antenna, a fixed or movable parabolic antenna or aconical antenna, and wave guides for guided wave radar application, suchas a still pipe, a transmission line or a probe, such as a single-lineprobe (including so-called Sommerfeld-probe or Goubau-probe), atwin-line probe or a coaxial probe.

Further, in the following description, embodiments of the presentinvention are mainly described with reference to an FMCW radar levelgauge using a stepped frequency sweep. It is noted that the presentinvention is advantageous in any sampled FMCW, such as a FMCW using acontinuous frequency sweep, or even in other types of radar systemsusing frequency sweeps.

FIG. 1 schematically illustrates a radar level gauge 1 according to anembodiment of the present invention, comprising a measurementelectronics unit 2, and a signal propagating device, here a horn antenna3. The radar level gauge 1 is provided on a tank 5, which is partlyfilled with a product 6 to be gauged. The tank 5 may be any container orvessel capable of containing a product, and may be metallic, or partlyor completely non-metallic, open, semi-open, or closed. The product 6 inthe tank may be a liquid, a liquid gas, or even a solid, such as grainor plastic pellets. The FMCW measurement method provides a relativelyhigh measurement sensitivity of the radar level gauge, enabling reliablemeasurement results also when interfering objects are present in thetank. By analyzing transmitted signal S_(T) being radiated by theantenna 3 towards the surface 7 of the product 6, and echo signal S_(R)traveling back from the surface 7, the measurement electronics unit 2can determine the distance between a reference position and the surface7 of the product 6, whereby the filling level L can be deduced. Itshould be noted that, although a tank 5 containing a single product 6 isdiscussed herein, the distance to any material interface present in thetank 5 can be measured in a similar manner. Furthermore the transmittedsignal S_(T) being radiated by the antenna 3 towards the surface 7,often comprises a frequency spectrum of 9 to 11 GHz or a frequencyspectrum of 24 to 27 GHz. In some cases, for example when using awaveguide and performing contact measurement the signal often comprisesa frequency spectrum of 1 to 3 GHz.

As is schematically illustrated in FIG. 1, the electronics unit 2comprises a transceiver 10 for transmitting and receivingelectromagnetic signals. The transceiver 10 is connected to the antenna3 via a connection line 9. The connection line 9 may be any suitablesignal medium, such as a coaxial cable or any electromagnetic waveguide.The unit 2 further comprises processing circuitry 11, which is connectedto the transceiver 10 for control of the transceiver 10 and processingof signal received by the transceiver 10 to determine the filling levelof the product 6 in the tank 5. The processing circuitry 11 is alsoconnected to a memory 12, storing any software required for theoperation of the radar level gauge 1, and also providing RAM used duringoperation.

The processing circuitry 11 is further connectable to externalcommunication lines 13 for analog and/or digital communication via aninterface 14. As an example, the communication between the communicationinterface 14 and an external control station (not shown) can be providedby a two-wire interface, which has a combined function of bothtransmitting the measurement result to the control station and receivingpower for operation of the gauge 1. Such a two-wire interface mayprovide a more or less constant power, and the measurement result can besuperimposed on the power voltage using a digital protocol, such asFieldbus Foundation, HART or Profibus. Alternatively, the current in thelines is regulated in accordance with the prevailing measurement result.An example of such an interface is the 4-20 mA industrial loop, wherethe current is regulated between 4 and 20 mA, depending on themeasurement result. Alternatively, the radar level gauge 1 maycommunicate wirelessly with the control station using e.g. a WirelessHART protocol, and use a local power supply (not shown) with batteriesor other means of scavenging energy for autonomous operation.

The interface 14 here includes power management circuitry, including apower store 15 for storing power during periods when the microwave unitis inactive, thereby enabling higher power consumption during periodswhen the microwave unit is active (i.e. during the sweep). With suchpower management, lower average power consumption may be achieved, whilestill allowing short periods of higher power consumption. The powerstore 15 may include a capacitance, and may be restricted by spacerequirements as well as intrinsic safety requirements (applying when thegauge 1 is arranged in the hazardous zone of a tank with explosive orflammable contents)

Although being shown as separate blocks in FIG. 1, several of thetransceiver 10, the processing circuitry 11 and the interface 14 may beprovided on the same circuit board, or even in the same integratedcircuit (IC).

Referring now to FIG. 2, there is shown a more detailed block diagram ofthe transceiver 10 and processing circuitry 11 in FIG. 2 according toone embodiment of the present invention.

The transceiver 10 here includes a microwave source 21, in turncontrolled by timing circuitry 23 forming part of the processingcircuitry 11. The microwave source 21 is connected to the antenna 3 viaa power divider 24, and also to a mixer 25. The power divider 24 isarranged to connect a return signal from the antenna to the mixer 25, inorder to allow the first mixer 25 to mix the transmitted signal from themicrowave source 21 with the return signal and provide the intermediatefrequency signal. The mixer 25 is connected to the filter arrangement26, which will filter the intermediate frequency signal to provide afiltered intermediate frequency signal. The filter arrangement is inturn is connected an amplifier 27.

The filter arrangement 26 will in various embodiments comprise at leasttwo high pass filters and at least one low pass filter. The filtercharacteristic of the filter arrangement using a first high pass filterat 3 kHz, a second high pass filter at 60 kHz and a low pass filter at100 kHz is shown in FIG. 3. Variations of the filter arrangement 26 areof course possible, and the frequencies may be adjusted according toeach application. In some embodiments a notch filter may be used tofilter out the lower frequencies.

The processing circuitry 11 here includes, in addition to the timingcircuitry 23 mentioned above, a sampler 31 adapted to receive and samplethe signal from the amplifier 27. The sampler may comprise asample-and-hold circuit in combination with an A/D-converter, or berealized as a sigma-delta converter. The sampler 31 may alsoadditionally comprise an anti-aliasing filter, configured to furtherdampen frequency portions of signals which are above half the samplingfrequency of the sampler 31. The sampler 31 is controlled by the timingcircuitry to be synchronized with the measurement signal. Therefore,based upon the measurement signal being a measurement sweep or adiagnostic sweep, the sampler 31 will send the sampled signal to a levelcalculator block 34, or a self-diagnosis block 35. The level calculatorblock 34 will determine the distance based on the sampled signal fromthe sampler 31 and the self diagnosis block 35 will self diagnose themicrowave chain and thus proof test the radar level gauge 1 based on thereceived sampled signal from the sampler 31. Generally, components whichtake part in generating, guiding, filtering, amplifying, transmittingand/or receiving electromagnetic microwaves are part of what isidentified as the microwave chain. Hence, in the exemplary embodimentsshown the microwave chain comprises at least the transceiver 10, as wellas the timing circuit 23 and the sampler 31. It should be noted that theself diagnosis block 35 and the level calculator block 35 mayfunctionally be one and the same unit.

While the elements of the transceiver 10 are typically implemented inhardware, and form part of an integrated unit normally referred to as amicrowave unit, at least some portions of the processing circuitry 11are typically embodied by software modules executed by an embeddedprocessor. The invention is not restricted to this particularrealization, and any implementation found suitable to realize the hereindescribed functionality may be contemplated.

The filter arrangement 26 and the filter characteristic of FIG. 3 willnow be described in conjunction with Fast-Fourier transformed (FFT)intermediate frequency signals of a measurement sweep and of adiagnostic sweep which are plotted in graphs of amplitude in decibelversus frequency in FIGS. 4A-C. Note the detection limit marked by thedotted line 40 in FIGS. 4A-C which marks at which level an echo isactually detectable.

The graph in FIG. 4A represents a tank spectrum without an effectivefilter arrangement 26. The reference echo 41 is seen as the first echoabove the detection limit, the second much larger echo 42 is the echoresulting from the connection to the antenna, hereafter antenna echo 42.Lastly, the much smaller echo 43 is actually the surface echo. Thus, itshould be understood that without an effective filter arrangement 26,the surface echo 43 will be very hard to distinguish from the referenceecho 41 and the antenna echo 42. In particular note that the surfaceecho 43 is relatively much closer to being below the detection limit 40.Further, the surface echo 43 in FIG. 4A represents a surface at adistance such that the echoes are separated for the sake of theexplanation. In many cases the surface echo 43 will have a lowerfrequency, representing a smaller distance and be completelyindistinguishable from the antenna echo 42.

The graph in FIG. 4B represents a measurement sweep, where compared toFIG. 4A, the filter arrangement 26 through a first high pass filter, isconfigured to suppress unwanted near zone echoes as the reference echo41. Note that the reference echo 41 is suppressed below the detectionlimit by the filter arrangement 26. This indicates that the referenceecho 41 will be impossible to detect during a measurement sweep. Theantenna echo 42 however is often strong enough to be detected above thedetection limit 40. The antenna echo 42 is different for each antennaand may degrade and change over time depending on the tank atmosphereand/or buildup of product on the antenna, therefore the antenna echo 42from the antenna connection is less suitable to use for the diagnosticfunctionality. The filter arrangement 26, through a second high passfilter, advantageously also compensates for the power loss as a functionof distance in free wave propagation, also known as free-space path losswherein the power decays according to an inverse square law. Thirdly,the filter arrangement 26, through a low pass filter limits thebandwidth of the signal to reduce the amount of noise which will beamplified by the amplifier 27. This bandwidth limit function of a lowpass filter comprised in the filter arrangement 26, may additionally actas an anti-aliasing filter and suppress frequencies which are higherthan half the sampling frequency of the processing circuitry 11. Therequirement for the filter arrangement to suppress the higherfrequencies will typically depend on the sampler 31. A sigma-deltaconverter may be driven at several MHz by an internal clock which is anorder of magnitude faster than the sampling frequency and thus requireno further suppression of high frequencies. Other types of samplers mayrequire a built-in low pass filter for anti-aliasing to further suppressthe frequencies. In at least one embodiment the sampler 31 is asuccessive approximation (SAR) ADC, and then may be oversampled and thusnot require any additional suppression of higher frequencies.

The anti-aliasing feature is indicated by the grayed out area startingat an anti-aliasing frequency 44 of the filter arrangement 26.

The graph in FIG. 4C represents a diagnostic sweep. Note that due to thehigher frequency now induced to the reference echo 45, the referenceecho 45 will be detectable above the detection limit as the filterarrangement 26 no longer suppresses the reference echo 45 to a largedegree. The antenna echo 46 is still present. As the reference echo 45is now detectable, it is also possible to use for a self-diagnosticfunction. However, note that during a diagnostic sweep the surface echo47 will have a higher frequency and therefore may even be above theanti-aliasing frequency 44, 48 of the filter arrangement. Thus, thesurface echo 47 may be filtered out by the low pass filter component ofthe filter arrangement 26. The surface echo 47 of FIG. 4C has been leftat the original size for the sake of understanding the connection withthe earlier figures. What occurs is that the surface echo 47, similar tothe reference echo 41 in the measurement sweep, will be suppressed belowthe detection limit 40. Hence, if the frequency of the surface echo 47is higher than the anti-aliasing frequency of the filter arrangement 26and the sampler 31 at the indicated frequency 48, measurement of thedistance L to the surface 7 may be rendered impossible during thediagnostic sweep.

The filtered intermediate frequency for the surface echo 43 in ameasurement sweep may be the same as the filtered intermediate frequencyfor the reference echo 45 in a diagnostic sweep. Thus in the case of ameasurement sweep the filtered intermediate frequency signal will beindicative of the distance to the surface, and in the diagnostic sweepthe filtered intermediate signal will be indicative of the distance tothe reference echo 45 which appear between the microwave source 21 andthe antenna 3.

In FIGS. 5A to 5C there are graphs showing frequency plotted versus timeand they represent the frequency sweeps generated by the microwavesource 21 and sent out from the antenna 3. The frequency for an echo isrelated to the frequency sweep according to the following equation:

$f = \frac{\lambda \cdot R \cdot h}{T \cdot c}$Where f is the frequency of the received intermediate frequency signal,c is the velocity of light in the present medium, h is the distance tothe echo, T is the sweep time and B is the sweep bandwidth. Thus, alonger distance h will provide an echo with a higher frequency. As thespeed of light and the distance to the echo are fixed, the presentinvention regards configuring the bandwidth B and/or sweep time T toperform a diagnostic sweep which enables the reference echo 45 to bedetectable above the detection limit.

In FIG. 5A a measurement sweep is shown with a bandwidth bw₁ and a sweeptime of t₁. The received signal having been mixed with the frequencysweep and filtered will produce a FFT similar to the one shown in FIG.4B.

In FIG. 5B a diagnostic sweep is shown with a bandwidth bw₁ and a sweeptime t₂ which is larger than t₁. The received signal having been mixedwith the frequency sweep and filtered will produce a FFT similar to theone shown in FIG. 4C.

In FIG. 5C a representative diagnostic sweep is shown with a bandwidthbw₂ which is larger than bw₁ and a sweep time t₁. The received signalhaving been mixed with the frequency sweep and filtered will produce aFFT similar to the one shown in FIG. 4C.

A combination of the representative frequency sweeps in FIGS. 5B and Cis of course also possible. It should now be readily apparent that byincreasing the ratio between the bandwidth and the sweep time in theabove equation the reference echo will have a higher frequency, whichmakes it detectable compared to an ordinary measurement sweep.

Now referring to FIG. 6, which is a flowchart illustrating a method forproviding a measurement of a distance to a surface of a productaccording to one embodiment of the present invention. First, the generalsteps S1-S6 of the method will be described, and then steps S11, S12,S61 and S62 will be elaborated upon.

First in step S1, the timing circuitry 23 controls the microwave source21 to output a signal in the form of a stepped frequency sweep. Thestepped frequency sweep is generated in the microwave source 21 whichcomprises a frequency stabilizing feedback loop. The frequencystabilizing feedback loop may in various embodiments be a phase lockedloop (PLL). The signal can normally be stepped from a lower frequency toa higher frequency in suitable steps. In an alternative embodiment thesignal may instead be stepped from a higher to a lower frequency. As anexample, the frequency sweep may have a bandwidth in the order of a fewGHz (e.g. 0.2-6 GHz), and an average frequency in the order of 25 GHz or10 GHz. This number of steps N in the sweep may be in the range100-4000, typically 200-2000, and may be around 1000 for a desired rangeof 30 m. The size of each frequency step (Δf) will thus typically be inthe order of MHz. For a power limited application the duration of thesweep is limited, and is typically in the order of 0-100 ms. As anexample, the duration of the measurement sweep may be around 5 ms, andwith 500 frequency steps (N=500), this results in a duration for eachstep equal to 10 μs, or an update rate of around 100 kHz. The durationof the steps are controlled by the sampling frequency of the frequencystabilizing feedback loop in the microwave source 21. Hence, theduration for the frequency steps are usually fixed, and in order toprovide a diagnostic sweep the number of steps and/or the start and stopfrequency of the sweep is controlled. Therefore, to provide a diagnosticsweep with a shorter sweep time, at the same bandwidth, means that thenumber of steps is reduced but that the size of each frequency step (Δf)will be larger. In a similar manner, to provide a diagnostic sweep witha larger bandwidth, in the same amount of time, means that the samenumber of steps are performed, but that the size of each frequency step(Δf) will be larger.

Secondly, in step S2, the signal from the microwave source 21 is guidedalong the propagation path from the microwave source 21 to the antenna3, and is emitted into the tank 5 as an electromagnetic transmit signalS_(T) by the antenna 3 towards the surface 7.

Then, in step S3, a return signal S_(R) traveling back from the surface7 after being reflected, is received by the antenna 3 and sent along thewaveguide 9 to the transceiver 10 and thus the power divider 24.

In step S4 the return signal S_(R) is sent via the power divider 24 tothe mixer 25 and is mixed with the signal to provide an intermediatefrequency signal. The intermediate frequency signal is a piecewiseconstant oscillating signal, with a frequency proportional to thedistance to the reflecting surface and the piecewise constant length isthe same length as the signals step length. A typical frequency is inthe order of kHz, e.g. less than 100 kHz, and typically less than 50kHz.

In step S5, the intermediate frequency signal from the mixer 25, isfiltered by the filter arrangement 26 which has been set allowintermediate frequency signals of a certain frequency to provide afiltered intermediate frequency. An exemplary characteristic of thefilter arrangement may be seen in FIG. 3. An amplifier 27 then amplifiesthe filtered intermediate frequency signal.

Then, in step S6, the amplified filtered intermediate frequency signalis received by the processing circuitry 11, where it is sampled andA/D-converted by the sampler 31. The sampling frequency of theA/D-converter 30 is advantageously sufficiently close to the update rateof the signal, in order to sample each step of the signal once and onlyonce. The sample vector resulting from the sampling is supplied to theself-diagnosis block 35 or the level calculator block 34 for furtherprocessing.

The timing circuitry 23 controls the microwave source 21 to generateeither a measurement sweep or a diagnostic sweep. During a measurementsweep the method continues from step S1 to step S11 wherein an ordinaryfrequency sweep as discussed above is generated and output, an exampleis shown in FIG. 5A. If the timing circuitry 23 controls the microwavesource 21 to generate a diagnostic sweep, the method would continue fromstep S1 to step S12 wherein a diagnostic sweep where the ratio betweenthe bandwidth and sweep time of the diagnostic sweep is adjusted inorder to enable echoes from the propagation path from the microwavesource 21 to the antenna 3 to act as reference echoes. Examples ofgenerated diagnostic sweeps are shown in FIGS. 5B and 5C.

Finally, the sampler 31 connected to the timing circuitry will eithercouple the sampled signal to the level calculator block 34 to performstep S61 of the method. Here the level calculator block 34 determinesthe frequency of the filtered intermediate frequency signal based on thesample vector, and then determines the distance to the reflectingsurface (and subsequently the filling level of the product in the tank)based on the frequency of the filtered intermediate frequency signal.Or, in the case of a diagnostic sweep, the sampler 31 will couple thesampled signal to the self-diagnosis block 35 to perform step S62 of themethod. Here the self diagnosis block 35 will determine if the radarlevel gauge is working properly. The self-diagnosis block 35 willcompare the measured reference echo against a stored profile of thereference echo comprising the expected distance and/or expectedamplitude. Hence, if the measured reference echo differs in any way fromthe stored value etc, the self-diagnosis block 35 will be able todetermine that a component in the microwave chain is faulty.

The radar level gauge 1 may perform the self-diagnosis of the microwavechain as often as necessary, or as often as an operator deems proper.For example, the radar level gauge 1 may perform a diagnostic sweep atleast once per hour or at least once per minute. Other intervals ofperforming diagnostic sweep are of course also possible, for instancesuch as performing diagnostic sweeps more or less often.

Although the figures may show a specific order of method steps, theorder of the steps may differ from what is depicted. Also two or moresteps may be performed concurrently or with partial concurrence. Suchvariation will depend on the software and hardware chosen and ondesigner choice. All such variations are within the scope of thedisclosure. Likewise, software implementations could be accomplishedwith standard programming techniques with rule based logic and otherlogic to accomplish the various connection steps, processing steps,comparison steps and decision steps. Additionally, even though theinvention has been described with reference to specific exemplifyingembodiments thereof, many different alterations, modifications and thelike will become apparent for those skilled in the art. For example, thefilter arrangement characteristic shown in FIG. 3 is one example theskilled addressee and person skilled in the art will easily understandthat other suitable filter characteristics may also be contemplated.Variations to the disclosed embodiments can be understood and effectedby the skilled addressee in practicing the claimed invention, from astudy of the drawings, the disclosure, and the appended claims.Furthermore, in the claims, the word “comprising” does not exclude otherelements or steps, and the indefinite article “a” or “an” does notexclude a plurality.

What is claimed is:
 1. A self-diagnosing FMCW radar level gauge formeasuring a distance to a surface of a product contained in a tank, saidradar level gauge comprising: a transceiver arranged to generate andtransmit an electromagnetic transmit signal in the form of a frequencysweep; a signal propagating device; a signal propagation path connectingsaid signal propagating device to said transceiver; wherein said signalpropagation path and said signal propagating device are configured toguide said electromagnetic transmit signal towards said surface, andreturn an echo signal including reflections from said surface and animpedance transition in the signal propagation path; a mixer connectedto said transceiver and configured to mix said echo signal with aportion of said electromagnetic transmit signal to provide anintermediate frequency signal; a filter arrangement connected to saidmixer and configured to filter said intermediate frequency signal inorder to provide a filtered intermediate frequency signal; andprocessing circuitry connected to said filter arrangement and configuredto process said filtered intermediate frequency signal; wherein saidfrequency sweep is one of a diagnostic sweep and a measurement sweep,said diagnostic sweep is configured such that a reference echo isdetectable in said filtered intermediate frequency signal, saidreference echo being indicative of a distance to said impedancetransition, said measurement sweep is configured such that saidreference echo is suppressed in said filtered intermediate frequencysignal, and that a surface echo is detectable in said filteredintermediate frequency signal, said surface echo being indicative of adistance to said surface; wherein said processing circuitry isconfigured to self-diagnose said radar level gauge based on thereference echo, and to determine the distance to said surface based onsaid surface echo.
 2. The radar level gauge according to claim 1,wherein said processing circuitry further comprises a self-diagnosisblock configured to compare said reference echo against a storedreference echo profile, said stored reference echo profile comprises anexpected distance to said reference echo and/or an expected amplitude ofsaid reference echo.
 3. The radar level gauge according to claim 1,wherein said diagnostic sweep has a shorter sweep time than themeasurement sweep.
 4. The radar level gauge according to claim 3,wherein the diagnostic sweep time is at least two times shorter than themeasurement sweep time.
 5. The radar level gauge according to claim 3,wherein the diagnostic sweep time is at least four times shorter thanthe measurement sweep time.
 6. The radar level gauge according to claim1, wherein said diagnostic sweep has a larger sweep bandwidth than themeasurement sweep.
 7. The radar level gauge according to claim 6,wherein the diagnostic sweep bandwidth is at least two times greaterthan the measurement sweep bandwidth.
 8. The radar level gauge accordingto claim 6, wherein the diagnostic sweep bandwidth is at least fourtimes greater than the measurement sweep bandwidth.
 9. The radar levelgauge according to claim 1, wherein said frequency sweep only comprisesfrequencies in the range 24 to 27 GHz.
 10. The radar level gaugeaccording to claim 1, wherein said frequency sweep only comprisesfrequencies in the range 1 to 3 GHz.
 11. The radar level gauge accordingto claim 1, wherein said frequency sweep only comprises frequencies inthe range 9 to 11 GHz.
 12. The radar level gauge according to claim 1,wherein said signal propagating device is one of a parabolic antenna, ahorn antenna, or a patch antenna.
 13. The radar level gauge according toclaim 1, wherein said signal propagating device is one of a sommerfeldprobe, a goubau probe, a coaxial probe, a twin-line probe or a stillpipe.
 14. The radar level gauge according to claim 1, wherein saiddiagnostic sweep is performed at least once per hour.
 15. The radarlevel gauge according to claim 1, wherein said diagnostic sweep isperformed at least once per minute.
 16. The radar level gauge accordingto claim 1, wherein said filter arrangement comprises at least two highpass filters, and at least one low pass filter.
 17. The radar levelgauge according to claim 16, wherein one of said at least two high passfilters is set at 3 kHz and another of said at least two high passfilters is set at 60 kHz and one of said at least one low pass filter isset at 100 kHz.
 18. A method for providing self-diagnosis of a FMCWradar level gauge for measuring a distance to a surface of a productcontained in a tank, said method comprising: generating anelectromagnetic transmit signal in the form of a frequency sweep;guiding said electromagnetic transmit signal via a signal propagationpath and a signal propagating device towards said surface; returning anecho signal including reflections from said surface and an impedancetransition in the signal propagation path; mixing said echo signal andsaid transmit signal to provide an intermediate frequency signal;filtering said intermediate frequency signal to provide a filteredintermediate frequency signal, wherein said generated frequency sweep isone of a diagnostic sweep and a measurement sweep, said diagnostic sweepis configured such that a reference echo is detectable in said filteredintermediate frequency signal, said reference echo being indicative of adistance to said impedance transition, said measurement sweep isconfigured such that said reference echo is suppressed in said filteredintermediate frequency signal, and that a surface echo is detectable insaid filtered intermediate frequency signal, said surface echo beingindicative of a distance to said surface; processing said filteredintermediate frequency signal to self-diagnose said radar level gaugebased on the reference echo, and to determine the distance to saidsurface based on said surface echo.
 19. The method according to claim18, wherein said step of processing said filtered intermediate frequencysignal to self-diagnose said radar level gauge comprises comparing saidreference echo against a stored reference echo profile, said storedreference echo profile including an expected distance to said referenceecho and/or an expected amplitude of said reference echo.
 20. The methodaccording to claim 18, wherein said diagnostic sweep has a shorter sweeptime than the measurement sweep.
 21. The method according to claim 20,wherein the diagnostic sweep time is at least two times shorter than themeasurement sweep time.
 22. The method according to claim 20, whereinthe diagnostic sweep time is at least four times shorter than themeasurement sweep time.
 23. The method according to claim 18, whereinsaid diagnostic sweep has a larger sweep bandwidth than the measurementsweep.
 24. The method according to claim 23, wherein the diagnosticsweep bandwidth is at least two times greater than the measurement sweepbandwidth.
 25. The method according to claim 23, wherein the diagnosticsweep bandwidth is at least four times greater than the measurementsweep bandwidth.
 26. The method according to claim 18, wherein saidfrequency sweep only comprises frequencies in the range 24 to 27 GHz.27. The method according to claim 18, wherein said frequency sweep onlycomprises frequencies in the range 1 to 3 GHz.
 28. The method accordingto claim 18, wherein said frequency sweep only comprises frequencies inthe range 9 to 11 GHz.